Printed electronics appear to hold a promise for enabling cost-effective integration of electronic functionality to a large variety of consumer products. Here it should be noted that even if a traditional, epoxy- or polyester based circuit board is often referred to as a printed circuit board (PCB), it does not fulfil the actual definition of printed electronics. In a PCB the use of (silk screen) printing is limited to producing the etch-resistant ink patterns prior to the etching of unwanted copper, as well as to producing visible markings on the surface of an otherwise completed board. True printed electronics mean that conductive, semiconductive, and/or possibly other patterns that constitute actual functional elements of the electronic circuit are formed on a substrate in a printing process. Although not mandatory, it is very advantageous if the process used to produce printed electronics is of the so-called roll-to-roll type, meaning that the substrate may come in the form of a long, rolled web, which is unwound for the printing step and can be again wound to a roll thereafter. Another widely used feeding mechanism is sheet feeding, in which the substrate comes in the form of a large number of sheets that are fed through the printing process.
A key question of producing printed electronics is how to ensure that conductive material is only distributed and attached to the desired portions of the substrate. A PCT application published with the publication number WO 2009/135985 contains a brief review of known methods, including electroplating, screen printing, flexo and rotogravure printing. Other known methods include inkjet printing and offset printing with conductive inks. Many of the known methods suffer from the drawback of requiring costly raw materials, like very fine-grained metal powder where the grain size is small enough not to clog spraying nozzles or the like.
Said PCT application discloses an improved method in which a substrate is first provided with patterns that attract conductive particles, by using either a spatially distributed electric charge or an adhesive or both. Conductive particles (which may in this case be much larger in size than in e.g. inkjet applications) are distributed onto the patterned substrate, so that they become attached only to the desired patterns. A sintering station with heated rolls is then used to sinter the particle-covered patterns into final conductive traces and areas that remain attached to the substrate. The PCT application published as WO 2009/135985 is incorporated herein by reference.
Although said improved method represents a clear advance compared to many previous methods, it leaves room for improvement in areas like peeling strength, continuity of conductivity, applicability of different conductive compounds and roll materials, as well as production speed.